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Moons of Neptune

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Shown in this image are Neptune and some of its moons: Triton, Galatea, Naiad, Thalassa, Despina, Proteus, and Larissa
An annotated picture of some ofNeptune's many moons as captured by theJames Webb Space Telescope.The bright bluediffraction starisTriton,Neptune's largest moon; whileHippocamp,its smallest regular moon, is too small to be seen.

The planetNeptunehas 16 knownmoons,which are named for minorwater deitiesand awater creatureinGreek mythology.[note 1]By far the largest of them isTriton,discovered byWilliam Lassellon 10 October 1846, 17 days after the discovery of Neptune itself. Over a century passed before the discovery of the second natural satellite,Nereid,in 1949, and another 40 years passed beforeProteus,Neptune's second-largest moon, was discovered in 1989.

Triton is unique among moons ofplanetary massin that its orbit isretrogradeto Neptune's rotation andinclinedrelative to Neptune's equator, which suggests that it did not form in orbit around Neptune but was instead gravitationally captured by it. The next-largest satellite in theSolar Systemsuspected to be captured,Saturn's moonPhoebe,has only 0.03% of Triton's mass. The capture of Triton, probably occurring some time after Neptune formed a satellite system, was a catastrophic event for Neptune's original satellites, disrupting their orbits so that they collided to form a rubble disc. Triton is massive enough to have achievedhydrostatic equilibriumand to retain a thin atmosphere capable of forming clouds and hazes.

Inward of Triton are seven smallregular satellites,all of which haveprograde orbitsin planes that lie close to Neptune's equatorial plane; some of these orbit amongNeptune's rings.The largest of them is Proteus. They were re-accreted from the rubble disc generated after Triton's capture after the Tritonian orbit became circular. Neptune also has eight more outerirregular satellitesother than Triton, including Nereid, whose orbits are much farther from Neptune and at high inclination: three of these have prograde orbits, while the remainder have retrograde orbits. In particular, Nereid has an unusually close and eccentric orbit for an irregular satellite, suggesting that it may have once been a regular satellite that was significantly perturbed to its current position when Triton was captured. Neptune's outermost moonS/2021 N 1,which has an orbital period of about 27Earth years,orbits farther from its planet than any other known moon in theSolar System.[1][2]

History[edit]

Discovery[edit]

Tritonwas discovered byWilliam Lassellin 1846, just seventeen days after thediscovery of Neptune.[3]Nereidwas discovered byGerard P. Kuiperin 1949.[4]The third moon, later namedLarissa,was first observed by Harold J. Reitsema, William B. Hubbard, Larry A. Lebofsky andDavid J. Tholenon 24 May 1981. The astronomers were observing a star's close approach to Neptune, looking for rings similar tothose discovered around Uranusfour years earlier.[5]If rings were present, the star's luminosity would decrease slightly just before the planet's closest approach. The star's luminosity dipped only for several seconds, which meant that it was due to a moon rather than a ring.

No further moons were found untilVoyager 2flew by Neptune in 1989.Voyager 2rediscovered Larissa and discovered five inner moons:Naiad,Thalassa,Despina,GalateaandProteus.[6]In 2001, two surveys using large ground-based telescopes found five additional outer irregular moons, bringing the total to thirteen.[7]Follow-up surveys by two teams in 2002 and 2003 respectively re-observed all five of these moons, which areHalimede,Sao,Psamathe,Laomedeia,andNeso.[7][8]The 2002 survey also found a sixth moon, but it could not be re-observed enough times to determine its orbit, and it thus becamelost.[7]

In 2013Mark R. ShowalterdiscoveredHippocampwhile examining Hubble Space Telescope images of Neptune'sring arcsfrom 2009. He used a technique similar topanningto compensate for orbital motion and allowstackingof multiple images to bring out faint details.[9][10]After deciding on a whim to expand the search area to radii well beyond the rings, he found an unambiguous dot that represented the new moon.[11]He then found it repeatedly in other archival HST images going back to 2004.Voyager 2,which had observed all of Neptune's other inner satellites, did not detect it during its 1989 flyby, due to its dimness.[9]

In 2021,Scott S. Sheppardand colleagues used theSubaru TelescopeatMauna Kea, Hawaiiand discovered two more irregular moons of Neptune, which were announced in 2024.[12]These two moons areprovisionally designatedS/2021 N 1andS/2002 N 5.The latter turned out to be a recovery of the lost moon from 2002.[2][13]

Discovery of outer planet moons

Moons of Jupiter
Moons of Saturn
Moons of Uranus
Moons of Neptune

Names[edit]

See also:Name conflicts with minor planets

Triton did not have an official name until the twentieth century. The name "Triton" was suggested byCamille Flammarionin his 1880 bookAstronomie Populaire,[14]but it did not come into common use until at least the 1930s.[15]Until this time it was usually simply known as "the satellite of Neptune". Other moons of Neptune are also named for Greek and Romanwater gods,in keeping withNeptune's position as god of the sea:[16]either fromGreek mythology,usually children ofPoseidon,the Greek equivalent of Neptune (Triton, Proteus, Despina, Thalassa); lovers of Poseidon (Larissa); other mythological creatures related to Poseidon (Hippocamp); classes of minor Greek water deities (Naiad,Nereid); or specific Nereids (Halimede, Galatea, Neso, Sao, Laomedeia, Psamathe).[16][17]

For the "normal" irregular satellites, the general convention is to use names ending in "a" for prograde satellites, names ending in "e" for retrograde satellites, and names ending in "o" for exceptionally inclined satellites, exactly like the convention for themoons of Jupiter.[18]Twoasteroidsshare the same names as moons of Neptune:74 Galateaand1162 Larissa.

Characteristics[edit]

The moons of Neptune can be divided into two groups:regularandirregular.The first group includes the seven inner moons, which follow circular prograde orbits lying in the equatorial plane of Neptune. The second group consists of all nine other moons including Triton. They generally follow inclined eccentric and often retrograde orbits far from Neptune; the only exception is Triton, which orbits close to the planet following a circular orbit, though retrograde and inclined.[19]

Orbit diagram of Neptune's inner moons including Triton, with their names and orbit directions indicated
Size comparison of Neptune's seven inner moons

Regular moons[edit]

In order of distance from Neptune, the regular moons areNaiad,Thalassa,Despina,Galatea,Larissa,Hippocamp,andProteus.All but the outer two are within Neptune-synchronous orbit(Neptune's rotational period is 0.6713 day or 16 hours[20]) and thus are beingtidally decelerated.Naiad, the closest regular moon, is also the second smallest among the inner moons (following the discovery of Hippocamp), whereas Proteus is the largest regular moon and the second largest moon of Neptune. The first five moons orbit much faster than Neptune's rotation itself ranging from 7 hours forNaiadandThalassa,to 13 hours forLarissa.

The inner moons are closely associated withNeptune's rings.The two innermost satellites, Naiad and Thalassa, orbit between theGalleandLeVerrier rings.[6]Despina may be ashepherd moonof the LeVerrier ring, because its orbit lies just inside this ring.[21]The next moon,Galatea,orbits just inside the most prominent of Neptune's rings, theAdams ring.[21]This ring is very narrow, with a width not exceeding 50 km,[22]and has five embedded brightarcs.[21]The gravity of Galatea helps confine the ring particles within a limited region in the radial direction, maintaining the narrow ring. Variousresonancesbetween the ring particles and Galatea may also have a role in maintaining the arcs.[21]

Only the two largest regular moons have been imaged with a resolution sufficient to discern their shapes and surface features.[6]Larissa, about 200 km in diameter, is elongated. Proteus is not significantly elongated, but not fully spherical either:[6]it resembles an irregularpolyhedron,with several flat or slightlyconcavefacets 150 to 250 km in diameter.[23]At about 400 km in diameter, it is larger than the Saturnian moonMimas,which is fully ellipsoidal. This difference may be due to a past collisional disruption of Proteus.[24]The surface of Proteus is heavily cratered and shows a number of linear features. Its largest crater, Pharos, is more than 150 km in diameter.[6][23]

All of Neptune's inner moons are dark objects: theirgeometric albedoranges from 7 to 10%.[25]Theirspectraindicate that they are made from water ice contaminated by some very dark material, probably complexorganic compounds.In this respect, the inner Neptunian moons are similar to the innerUranian moons.[6]

Irregular moons[edit]

The orbit ofTriton(red) is different from most moons' orbit (green) in the orbit's direction, and the orbit istilted −23°.

In order of their distance from the planet, the irregular moons areTriton,Nereid,Halimede,Sao,S/2002 N 5,Laomedeia,Psamathe,Neso,andS/2021 N 1,a group that includes both prograde and retrograde objects.[19]The seven outermost moons are similar to the irregular moons of othergiant planets,and are thought to have been gravitationally captured by Neptune, unlike the regular satellites, which probably formedin situ.[8]

Triton and Nereid are unusual irregular satellites and are thus treated separately from the other seven irregular Neptunian moons, which are more like the outer irregular satellites of the other outer planets.[8]Firstly, they are the largest two known irregular moons in the Solar System, with Triton being almost anorder of magnitudelarger than all other known irregular moons. Secondly, they both have atypically small semi-major axes, with Triton's being over an order of magnitude smaller than those of all other known irregular moons. Thirdly, they both have unusual orbital eccentricities: Nereid has one of the most eccentric orbits of any known irregular satellite, and Triton's orbit is a nearly perfect circle. Finally, Nereid also has the lowest inclination of any known irregular satellite.[8]

Triton[edit]

Main article:Triton (moon)
Irregular satellites of Jupiter (red), Saturn (green), Uranus (magenta) and Neptune (blue; including Triton), plotted by distance from their planet (semi-major axis) in the horizontal axis andorbital inclinationin the vertical axis. The semi-major axis values are expressed as a fraction of the planet'sHill sphere's radius, while the inclination is expressed indegreesfrom theecliptic.The relative sizes of moons are indicated by the size of their symbols, and the Sao and Neso groups of Neptunian moons are labeled. Data as of February 2024.

Triton follows a retrograde and quasi-circular orbit, and is thought to be a gravitationally captured satellite. It was the second moon in the Solar System that was discovered to have a substantialatmosphere,which is primarilynitrogenwith small amounts ofmethaneandcarbon monoxide.[26]The pressure on Triton's surface is about 14μbar.[26]In 1989 theVoyager 2spacecraft observed what appeared to be clouds and hazes in this thin atmosphere.[6]Triton is one of the coldest bodies in the Solar System, with a surface temperature of about 38 K (−235.2 °C).[26]Its surface is covered by nitrogen, methane,carbon dioxideand waterices[27]and has a highgeometric albedoof more than 70%.[6]TheBond albedois even higher, reaching up to 90%.[6][note 2]Surface features include the large southernpolar cap,older cratered planes cross-cut bygrabenandscarps,as well as youthful features probably formed by endogenic processes likecryovolcanism.[6]Voyager 2observations revealed a number of activegeyserswithin the polar cap heated by the Sun, which eject plumes to the height of up to 8 km.[6]Triton has a relatively high density of about 2 g/cm3indicating thatrocksconstitute about two thirds of its mass, and ices (mainly water ice) the remaining one third. There may be a layer of liquid water deep inside Triton, forming a subterranean ocean.[28]Because of its retrograde orbit and relative proximity to Neptune (closer than the Moon is to Earth),tidal decelerationis causing Triton to spiral inward, which will lead to its destruction in about 3.6 billion years.[29]

Nereid[edit]

Main article:Nereid (moon)

Nereid is the third-largest moon of Neptune. It has a prograde but very eccentric orbit and is believed to be a former regular satellite that was scattered to its current orbit through gravitational interactions during Triton's capture.[30]Water ice has been spectroscopically detected on its surface. Early measurements of Nereid showed large, irregular variations in its visible magnitude, which were speculated to be caused by forcedprecessionor chaotic rotation combined with an elongated shape and bright or dark spots on the surface.[31]This was disproved in 2016, when observations from theKepler space telescopeshowed only minor variations. Thermal modeling based on infrared observations from theSpitzerandHerschelspace telescopes suggest that Nereid is only moderately elongated which disfavours forced precession of the rotation.[32]The thermal model also indicates that the surface roughness of Nereid is very high, likely similar to the Saturnian moonHyperion.[32]

Nereid dominates the normal irregular satellites of Neptune, having about 98% of the mass of Neptune's entire irregular satellite system altogether (if Triton is not counted). This is similar to the situation ofPhoebeat Saturn. If it is counted as a normal irregular satellite (but not Triton), then Nereid is also by far the largest normal irregular satellite known, having about two-thirds the mass of all normal irregular moons combined.[33]

Normal irregular moons[edit]

Among the remaining irregular moons, Sao, S/2002 N 5, and Laomedeia follow prograde orbits, whereas Halimede, Psamathe, Neso and S/2021 N 1 follow retrograde orbits. There are at least two groups of moons that share similar orbits, with the prograde moons Sao, S/2002 N 5, and Laomedeia belonging to the Sao group and the retrograde moons Psamathe, Neso, and S/2021 N 1 belonging to the Neso group.[12]The moons of the Neso group have the largest orbits of any natural satellites discovered in the Solar System to date, with average orbital distances over 125 times the distance between Earth and the Moon and orbital periods over 25 years.[34]Neptune has the largestHill spherein the Solar System, owing primarily to its large distance from the Sun; this allows it to retain control of such distant moons.[19]Nevertheless, the Jovian moons in theCarmeandPasiphae groupsorbit at a greater percentage of their primary's Hill radius than the Neso group moons.[19]

Formation[edit]

The mass distribution of the Neptunian moons is the most lopsided of the satellite systems of thegiant planetsin the Solar System. One moon, Triton, makes up nearly all of the mass of the system, with all other moons together comprising only one third of one percent. This is similar to the moon system of Saturn, whereTitanmakes up more than 95% of the total mass, but is different from the more balanced systems of Jupiter and Uranus. The reason for the lopsidedness of the present Neptunian system is that Triton was captured well after the formation of Neptune's original satellite system, and experts conjecture much of the system was destroyed in the process of capture.[30][35]

The relative masses of the Neptunian moons

Triton's orbit upon capture would have been highly eccentric, and would have caused chaotic perturbations in the orbits of the original inner Neptunian satellites, causing them to collide and reduce to a disc of rubble.[30]This means it is likely that Neptune's present inner satellites are not the original bodies that formed with Neptune. Only after Triton's orbit became circularised could some of the rubble re-accrete into the present-day regular moons.[24]

The mechanism of Triton's capture has been the subject of several theories over the years. One of them postulates that Triton was captured in athree-bodyencounter. In this scenario, Triton is the surviving member of abinaryKuiper beltobject[note 3]disrupted by its encounter with Neptune.[36]

Numerical simulations show that there is a 0.41 probability that the moon Halimede collided with Nereid at some time in the past.[7]Although it is not known whether any collision has taken place, both moons appear to have similar ( "grey" ) colors, implying that Halimede could be a fragment of Nereid.[37]

List[edit]

Orbital diagram of theorbital inclinationand orbital distances for Neptune's rings and moon system at various scales. Notable moons and rings are individually labeled. Open the image for full resolution.

The Neptunian moons are listed here by orbital period, from shortest to longest. Irregular (captured) moons are marked by color. The orbits and mean distances of the irregular moons are variable over short timescales due to frequent planetary and solarperturbations,therefore the listed orbital elements of all irregular moons are averaged over a 30,000-year period: these may differ fromosculatingorbital elements provided by other sources.[38]Their orbital elements are all based on theepochof 1 January 2020.[1]Triton, the only Neptunian moon massive enough for its surface to havecollapsedinto aspheroid,is emboldened.

Key

Inner moons 		♠
Triton 		†
Nereid 		‡
Halimede 		♦
Sao group 		♥
Neso group
Neptunian moons
Label
[note 4] 		Name Pronunciation
(key) 		Image Abs.
magn. 		Diameter
(km)[note 5] 		Mass
(×1016kg)
[note 6] 		Semi-major axis
(km)[17] 		Orbital period
(d)[1] 		Orbital inclination
(°)[1] 		Eccentricity
[17][note 7] 		Discovery
year[16] 		Year announced Discoverer
[16] 		Group
III Naiad /ˈneɪəd,ˈnaɪæd/[42]
A smeared white object elongated from the bottom-left to top-right can be seen in the center.
 9.6 60.4
(96 × 60 × 52)		 ≈ 13 48224 +0.2944 4.691 0.0047 1989 1989 Voyager Science Team inner
IV Thalassa /θəˈlæsə/
A group of three objects, each circled and labeled by the respective designations. Thalassa is the central object designated 1989 N5.
 8.7 81.4
(108 × 100 × 52)		 ≈ 35 50074 +0.3115 0.135 0.0018 1989 1989 Voyager Science Team inner
V Despina /dəˈspaɪnə/
A white oval shaped object somewhat elongated horizontally is seen in the center. There are a few small dark spots on its surface.
 7.3 156
(180 × 148 × 128)		 ≈ 170 52526 +0.3346 0.068 0.0004 1989 1989 Voyager Science Team inner
VI Galatea /ˌɡæləˈtiːə/
A small white object elongated from the bottom-left to top-right can be seen in the center.
 7.2 174.8
(204 × 184 × 144)		 ≈ 280 61953 +0.4287 0.034 0.0001 1989 1989 Voyager Science Team inner
VII Larissa /ləˈrɪsə/
An irregularly shaped grey object slightly elongated horizontally occupies almost the whole image. Its surface shows a number of dark and white spots.
 6.8 194
(216 × 204 × 168)		 ≈ 380 73548 +0.5555 0.205 0.0012 1981 1981 Reitsema et al. inner
XIV Hippocamp /ˈhɪpəkæmp/
Composite of multiple Hubble images of the Neptune system, with the moons appearing as bright white dots. The fainter dot to the upper right is Hippocamp, circled and labeled to distinguish it from other moons in this image.
 10.5 34.8±4.0 ≈ 2.2 105283 +0.9500 0.064 0.0005 2013 2013 Showalteret al. inner
VIII Proteus /ˈproʊtiəs/
A conically shaped object is seen almost fully illuminated from the left. The cone axis looks towards the observer. The outline of the object is a rectangle with rounded corners. The surface is rough with a few large depressions.
 5.0 420
(436 × 416 × 402)		 ≈ 3900 117646 +1.1223 0.075 0.0005 1989 1989 Voyager Science Team inner
I Triton♠ /ˈtraɪtən/
A large spherical object is half-illuminated from the bottom-left. The south pole faces to the light source. Around it in the bottom-left part of the body there is a large white area with a few dozens dark streaks elongated in the pole to equator direction. This polar cap has a slight red tinge. The equatorial region is darker with a tint of cyan. Its surface is rough with a number of craters and intersecting lineaments.
 –1.2 2705.2±4.8
(2709 × 2706 × 2705)		 2139000 354759 −5.8769 156.865 0.0000 1846 1846 Lassell
II Nereid† /ˈnɪəriəd/
A small white smeared body is seen in center.
 4.4 357 ± 13 ≈ 2400 5504000 +360.14 5.8 0.749 1949 1949 Kuiper
IX Halimede‡ /ˌhæləˈmiːdiː/
10.0 ≈ 62 ≈ 12 16590500 −1879.78 119.6 0.521 2002 2003 Holmanet al.
XI Sao♦ /ˈseɪoʊ/
11.1 ≈ 44 ≈ 3.4 22239900 +2919.43 50.2 0.296 2002 2003 Holman et al. Sao
S/2002 N 5♦
11.2 ≈ 38 ≈ 3 23414700 +3156.56 46.3 0.433 2002 2024 Holman et al. Sao
XII Laomedeia♦ /ˌleɪəməˈdiːə/
10.8 ≈ 42 ≈ 3.4 23499900 +3176.13 36.9 0.419 2002 2003 Holman et al. Sao
X Psamathe♥ /ˈsæməθiː/
11.0 ≈ 40 ≈ 2.9 47615100 −9149.51 127.8 0.414 2003 2003 Sheppardet al. Neso
XIII Neso♥ /ˈniːsoʊ/
10.7 ≈ 60 ≈ 11 49895300 −9794.99 128.4 0.455 2002 2003 Holman et al. Neso
S/2021 N 1♥ 12.1 ≈ 25 ≈ 0.8 50700200 −10036.65 135.2 0.503 2021 2024 Sheppard et al. Neso

See also[edit]

Notes[edit]

  1. ^This is a IAU guideline that will be followed at the naming of every Neptunian moon, although two (S/2002 N 5andS/2021 N 1) have yet to receive permanent names.
  2. ^The geometric albedo of an astronomical body is the ratio of its actual brightness at zerophase angle(i.e. as seen from the light source) to that of an idealized flat, fully reflecting,diffusively scattering(Lambertian) disk with the same cross-section. The Bond albedo, named after the American astronomerGeorge Phillips Bond(1825–1865), who originally proposed it, is the fraction ofpowerin the totalelectromagnetic radiationincident on an astronomical body that is scattered back out into space. The Bond albedo is a value strictly between 0 and 1, as it includes all possible scattered light (but not radiation from the body itself). This is in contrast to other definitions ofalbedosuch as the geometric albedo, which can be above 1. In general, though, the Bond albedo may be greater or smaller than the geometric albedo, depending on surface and atmospheric properties of the body in question.
  3. ^Binary objects,objects with moons such as thePluto–Charonsystem, are quite common among the largertrans-Neptunian objects(TNOs). Around 11% of all TNOs may be binaries.[36]
  4. ^Label refers to theRoman numeralattributed to each moon in order of their discovery.[16]
  5. ^Diameters with multiple entries such as "60×40×34" reflect that the body is not spherical and that each of its dimensions has been measured well enough to provide a 3-axis estimate. The dimensions of the five inner moons were taken from Karkoschka, 2003.[25]Dimensions of Proteus are from Stooke, 1994.[23]Dimensions of Triton are from Thomas, 2000,[39]whereas its diameter is taken from Davies et al., 1991.[40]The size of Nereid is from Kiss et al., 2016,[32]and the sizes of the other outer moons are from Sheppard, with the diameters of S/2002 N 5 and S/2021 N 1 calculated assuming an albedo of 0.04.[34]
  6. ^Of all known moons of Neptune, only Triton has a reliably measured mass.[41]The masses of all regular satellites were estimated by JPL,[41]while all other irregular moons of Neptune were calculated assuming a density of 1 g/cm3.
  7. ^Since the reference Showalter et al. (2019) does not cover irregular moons (with colored background), their eccentricities are taken from Planetary Satellite Mean Elements of JPL.[1]

References[edit]

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  13. ^"MPEC 2024-D114: S/2002 N 5".Minor Planet Electronic Circular.Minor Planet Center. 23 February 2024.Archivedfrom the original on 3 March 2024.Retrieved23 February2024.
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